Jiggle Physics, also known as Soft-Body Physics, or sometimes Cloth Physics, is the art and science of simulating the behavior of non-rigid objects and materials in a computer. In this respect, it's a sister trope to Ragdoll Physics, which deals with more rigid structures.

The earliest uses of Jiggle Physics were not for simulating anatomy, but rather for hair and clothing. Of course, the computing power to simulate soft anatomy became commercially available much sooner, so for a relatively cheap gimmick it could be applied to body fat and the example which first comes to mind for most people when they hear the term, breasts.

Next time you play a game or watch an all-CGI movie or cartoon, take a look at what bounces and jiggles in reaction to physical stimuli. Whether it's the Badass Longcoat of the hero bouncing and flapping around as he runs, the gelatinous monster wobbling as it's pumped full of plasma fire, or Ms. Fanservice putting on a (perhaps unintentional) show, there's some pretty clever programming behind it.

For a more low-tech, more specific Sister Trope, see Gainaxing. Examples from sprite-based games should go there as they do not involve physics computations. Implementing Jiggle Physics almost always means an aversion of No Flow in CGI.

Examples:

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Sports bra manufacturers Shock Absorber used jiggle physics to create what they called the Bounce-O-Meter, an onscreen device by which a woman could graphically see what level of stress a variety of sports put her breasts to. After selecting cup size and activity level, the screen opens up with a graphic view of heaving breasts, with a standard bra, a restraining bra, and no bra. (NSFW in a respectable way.)

The latest of the Tomb Raider games have it, as would seem obvious. The early games show exactly what is wrong with its absence; Lara's chest is literally Barbie-esque in its steely inflexibility, as if she were smuggling a pair of cannonballs in her bodysuit, or a pair of traffic cones if you played the first game. It was only in the cutscenes for Legend and Anniversary; Underworld has it in the gameplay itself. It's very subtle, but it is there. At least unless you use a user-made Jiggle+ PC mod for Underworld, which approaches Gainaxing.

In the latest games, the developers actually used the jiggle physics from Dead or Alive, below, for use in Lara's ponytail.

In Ōkami, wood-sprite Sakuya and priestess Rao both take full advantage of a physics system which was designed to show off everything that can bounce and sway. You know, their hair. (And busts.) While Sakuya is an Innocent Fanservice Girl, Rao is most definitely doing it on purpose.

The Legend of Zelda: The Wind Waker was the first game in the Zelda series to have a full-blown physics system for animating hair, clothes, flags, and grass, though The Legend of Zelda: Majora's Mask was the first in the series to have some type of jiggle physics (for a few of the masks). In The Wind Waker, the jiggle physics allow for some spectacular billowing effects, especially with Ganon's cloak during the final battle of the game.

The Dead or Alive series is known for having it, playing with it, and selling the game with it. An age input on the second title actually determined how much the female fighters could bounce. The older your age, the more they did, up to a point that a 99-year-old man probably wouldn't be able to play without a heart attack. Even while standing still they tend to bounce. A later Spin-Off would bring it further to the forefront by transforming the game into a beach volleyball title and eliminating the male competitors. Their hair is also lovingly animated and streams all over the place as they move. Essentially this is one of Tomonobu Itagaki's defining traits of his games and is also highly prevalent in Ninja Gaiden, most egregiously in Ninja Gaiden Σ2, where you can affect Ayane's assets by shaking your controller up and down, and side-to-side.

In the Nintendo 3DS version, shaking the system while viewing collectible figurines of female characters will cause their breasts to bounce in relation to how you shook the system. And to think that originally, Nintendo wasn't going to equip the system with motion control at all...

The latest Dead or Alive installments (Dead or Alive 5 Plus for the Playstation Vita and Dead or Alive 5 Ultimate for the PlayStation 3) now put the Jiggle Physics in your hands as well, taking after the feature in Ninja Gaiden Sigma 2, letting you manually jiggle any female character's breasts by swiping the PS Vita's screen in DOA5+ or jiggling the Dualshock 3 controller in DOA5U (after activating the Breast Physics' "OMG" mode in the latter's game settings).

Dead or Alive 5: Last Round makes use of the new "Soft Engine", a secondary physics engine entirely dedicated to animating breasts. They don't just jiggle, though; they compress and deform if the forces applied call for it. From a technical standpoint, it's genuinely impressive.

This is practically the entire point of the Senran Kagura series. Interestingly, though, there's no actual physics there. The games you pre-set animations dependent on what else the model is doing, but there's enough variety you hardly notice.

The Soul Series games display this, but it is very much downplayed and almost, dare say, "realistic", at least early on in the series. Still, it has many varying examples.

The ninja Taki, whose breast movements, while they don't bounce too much, appear to be timed one full second behind the rest of her body.

Xianghua, though, has buttock jiggle physics. So does Talim, and maybe others.

Of course this was all thrown out in SoulCalibur 3, in which all female character with the exception of the underaged Talim have been given larger breasts and costumes that allow these to bounce around in a ridiculous manner, such as when you SELECT them in the character selection bit. Astaroth's chest jiggle is still intact. If you leave the shop while one of the storekeepers is gesturing, their boobs jiggle up at a ridiculous height, and then go back to normal.

By the time of the fourth game, cover-art character Ivy's move with so much momentum it's stunning the rest of her body doesn't follow them into a knock-out when she stops moving. It's considered part of the package deal, alongside destructible clothing.

In Injustice: Gods Among Us, Batman's Super involves attacking his enemy with two tazers, causing them to spasm uncontrollably. This presents some issues with more cleavage-y characters like Harley Quinn.

Smackdown VS Raw 2011 has this with their divas, in both the breast area, and the buttocks region.

In Lightning Legend: Daigo no Daibouken, this is the main animation feature of Misa Atago, the resident busty female character, to the point one's can wonder her breasts manage to stay in those Impossibly-Low Neckline clothes with all that bouncing. She's also the only female character having those physics, the other girls being the flat-chested Mayu Uzaka and the young Yuki Shirogane.

UFC Undisputed has this for the ring girls Ariani and Shandella.

Bloody Roar: Primal Fury was particularly egregious with this. EVERY character had jiggle physics - the women in obvious places, but the men's biceps would bounce as well. Apparently nothing below the neck was actually connected to the characters' skeletons.

Virtua Fighter5 uses this. The breast size and the type of clothing the female character wears actually affects the amount of jiggle, making this a much more realistic and well executed example than most. The only character this doesn't effect is Eileen, the youngest girl in the cast. Inversely, Vanessa, who has Boobs of Steel, jiggles no matter what she wears.

Jigglebones were added to the glove on the Pyro's hat. Considering the Pyro is possibly a girl, that's the closest you'll get to Jiggle Physics on a Team Fortress 2 character. Unless you download one of thecharacter mods that converts Pyro into a girl. Well, a less ambiguous 'girl'.

Some other hats have also received this treatment since, including the Hustlers Hallmark (a pimp hat for the demoman, only for the feather) and the Pyro Brainslug.

Left 4 Dead adds jiggle physics to the Boomer, a extremely bloated zombie whose belly has grown out of the shirt. The sequel adds a female version of the Boomer with similar results in the jiggle. It was also used for Zoey's... ponytail.

Halo initially had none of this, but Halo: Reach added subtle jiggle physics on Spartans' rear ends, and Halo 4 added this on female Spartans' breasts. However, the latter is quite difficult to notice; walking with a ripped-off mounted turret is the easiest way to see it.

Kasuga from Sengoku Basara (a kunoichi sporting a... veryrevealingoutfit) originally lacked jiggle physics in the first few games. It was eventually added in Samurai Heroes (where she was unfortunately downgraded to an NPC), though it was actually quite subtle and realistic.

Samurai Heroes also gives us Magoichi Saika, the gun-toting leader of the Saika Faction, garbed in a metal corset. When you position the camera to face her front side (to make it more visible, have Magoichi stand against a wall, which forces the camera to zoom in while positioning it), you'll notice that her breasts jiggle while moving, and slightly more so after dashing. Her breasts will also jiggle while performing her taunt, which is easier to notice since you can just station the camera in front of her without fighting to keep it positioned that way. Have fun.

The modern Ninja Gaiden games - as made by the same team behind Dead or Alive - feature several fine ladies that bounce to almost comedic levels. In fact, in Ninja Gaiden Sigma II, you can shake the controller to make all present boobies bounce to your leisure.

No More Heroes 2: Desperate Struggle has them. However, instead of just being used for breasts, they are also used for things such as Travis beam katanas dangling from his belts, his jacket, and his hair.

Dead Rising has this on the manboobs and spare tire gut of Larry, a Fat Bastard boss. One order of Brain Bleach, coming up. Meanwhile, Jessica is the only woman in the game who has it as well, just because her breasts are so big.

Every time the player stops running in Haunting Ground, the female protagonist's breasts bounce, despite not doing so when she runs. They also jiggle quite a bit even when she turns around. That, coupled with the convenient close-ups of Fiona's upper torso when she falls, makes the game a big dose of Fanservice.

In Dead Space, it's most noticeable with Kendra's hair. Jiggle physics, however, doesn't prevent that hair from flying through her body occasionally.

In the fourth Fatal Frame game, Ruka's butt jiggles when she has the optional Zero Suit equipped.

Platformer

Rouge the Bat in Sonic Adventure 2 has this. Yes, they seriously used this on the breasts (and ears, note that they move in time with her chest) of a cartoon bat in a kid's video game. The titular protagonist and his Evil Doppelganger have this applied to their quills as well.

The Floor is Jelly has this applied to the game world you explore, with the ground and walls fluidly wobbling and jiggling as you jump around. Even background objects such as trees and bushes bounces along with the ground!

Project Diva F has this for Meiko, although it's more noticeable in her song "Nostalogic".

RPG

Seen in Dragon Quest VIII with the character Jessica. She has enormous boobs, which jiggle and bounce whenever she does anything, several different character models of her in skimpy clothing, and even a skill set called "Sex Appeal". Lampshaded in the game, when her mother chastises her for her choice of revealing traveling apparel.

The game Vampire: The Masquerade – Bloodlines has numerous female models for the player to control, depending on what type of vampire he/she chooses. Oddly, only some of these have jiggle physics - others appear to have breasts made of solid metal. This makes sense, once you consider that the game is set in L.A., and that some of those characters have very tight dresses/tops, whereas others Malkavian are more like a loose band of cloth. The "money" cheat lets you edit size, just in case you needed to check. They developed a version of this technology, and were the ones who coined the term, 'Jiggle Physics'.

The Steam release of Final Fantasy IX gave this to Beatrix. Her old model was completely rigid, but now every time she claps a hand over her chest in a show of loyalty, well...

In Final Fantasy X, busty gothic lolita-styled mage Lulu had breasts that moved as if they were filled with water. If you ended a battle with her hasted, they appeared to jump like a prodded frog.

In Final Fantasy XII, Fran's buttocks do this at several points, most notably when she walks away from other Viera in Eryut Village.

There are a few cutscenes where Tifa does this in Final Fantasy VII. Tifa's infamous knockers were caused by rendering limitations on the Playstation. The original character was supposed to have B- or C-cup breasts (as seen in concept art), but when the graphics programmers realized the issues at hand, it boiled down to huge knockers, or flat as a board. The team chose the former.

Ironically, the Bioware RPG game Neverwinter Nights had only a single character model with jiggle physics - a female wearing a bare-shouldered Breast Plate. The armour was solid, but apparently the breasts behind it weren't attached to the woman, or snug against the armour. When fan-content added jiggle physics to other character models, even the most rigid steel full plate armour jiggled like a jelly castle. Most fan-made jiggle mods have boobs that jiggle in the wind. Other than that, A Dance with Rogues Jiggle Physics made a bit of sense (all the outfits the character would likely be wearing were rather...exposed).

Dragon Age II has major Jiggle Physics for the breasts of any female character not wearing plate armor, as well as some longer hair.

World of Warcraft features jiggle physics. Note that they were mostly removed from the Warlords of Draenor expansion's new character models, but the old models can still be toggled.

Most female playable-characters in the game have some form of jiggle, but Female Tauren and Trolls have it particularly blatant - their idle animation has them stand there breathing while their huge boobs bounce up and down like they're busy on a trampoline. The female Dwarf has jiggle just from walking!

The female night elves are even more blatant. Their most common idle animation is to hop up and down on their toes several times. Boingyboingy! Their cowering/fear animation makes the bounciness especially obvious.

Depending on your personal taste, one of the Human female's idle animations could count as either an attempt at this or outright Fan Disservice - she just stands there, taking a few deep breaths - but the model's way of animating this is to keep her chest be stock-still and have her breasts subtly-but-visibly inflate and deflate instead.

Judith from Tales of Vesperia is guilty of this. In the PS3 remake, when performing her Second Mystic Arte, her animated-cut shows her hunnies jiggling for about a split-second as she throws her spear. Other few Tales characters, like Sheena from Tales of Symphonia, have it too, but it is not obvious unless you watch them move very carefully and up close.

Neptunia has jiggle physics for, not only the in game models but also the CG cutescenes and models!. It's more noticable if you check the CG Gallery and move it around. Notably, flat characters like Blanc/White Heart or IF have jiggling breasts, too.

Any female character who had a noticeable bust in Agarest Senki 2 did these in HD sprites.

Similarly, there is mod for Skyrim called 'BBP' that adds breast physics, which, when combined with another mod (CBBE), can be easily used to create Gag Boobs (or the opposite if you so choose).

Star Ocean: The Last Hope has Myuria, whose breasts wobble all around the place even when she's standing still (as evidenced by the cutscene in which she's first fully revealed). This provides an answer to why Reimi keeps her assets so securely bound in place...

...Though Reimi's backside might be another story, as it shows signs of jiggling whenever she does any sort of action during battle, but one must be quick to notice when she's zoomed in.

Shadow Hearts Covenant has Karin bouncing along to jiggle physics when she runs, but it's not her chest, it's her hair. The straps on Yuri's bag also seem to have jiggle bones in them and flap along behind him in response to his movements.

Vindictus takes full advantage of Valve's physics engine to incorporate some rather... ludicrous movements to that area.

The Epic Battle Fantasy series has the character Natalie, a mage who's boobs bounce during the victory animation. If you click on them enough times, you get an award.

A decent fraction of all the animation in Global Agenda is in making the medic's goo tubes jiggle.

In Scarlet Blade has attached jiggle physics to it's all female cast as part of the gratuitous fanservice it features.

Dungeon Fighter Online has jiggle physics on the female Fighter class, as well as the Thief class (which is exclusively female). Some female NPCs breasts move when they are standing stock-still.

Handled realistically in Final Fantasy XIV, where it's dependent on the outfit being used, and subtle even when present. Outfits that are revealing and/or visibly lacking support like the harnesses or the female Miqo'te starting garb predictably have the most, while any proper leather or plate armor tends to be totally rigid.

Used for hair, clothing and chests in Phantasy Star Online 2. In the case of the latter of the three, it's actually fairly subtle outside of numerous emotes that seem specifically designed to show it off.

Spore. Oh god... With the right combination of body parts, anything can jiggle.

Jiggle Physics were recently added to Second Life as well. As with all things Second Life, it can be manipulated and tweaked to your heart's content to create things that occasionally require Brain Bleach.

The Gainax-y flavour of the jiggle is rather enhanced by the fact that things like teleporting count as a movement that results in that kind of jiggle, as does sitting (because you basically teleport into the sitting position rather than moving naturally).

Metal Gear Acid 2 came with a pair of 3D goggles allowing you to see the breasts' undulatory movements in gropeable solidarity. To save time, the developers used the same physics set on the ludicrous ammo pouch Snake wore on his own chest. The effect was...well... Not to mention that Venus, when, getting bored or some else, would hop up and down. WHY, HELLO, THERE. Also, due to how avatars are typically added when a new area is loaded everyone's breasts/ammo pouches would simultaneously, spontaneously bounce up and down. To prevent problems with e.g. someone's feet intersecting/sinking into the ground, they are actually created in the air and allowed to drop a short distance to the ground. Rendering can be delayed a second to hide the drop, but the afterjiggle...

For some really, really well-done Jiggle Physics, look at EVA from Metal Gear Solid 3. Unless you're paying attention, you won't notice - her breasts move exactly like a real person's. Particularly of note is the scene where, clad in her underwear, she crawls hands and knees towards Snake as part of an attempted seduction, and her breasts actually change shape - again, exactly like a real person's. The irony kicks in when it turns out that the woman has had bust enhancement surgery, which, using the techniques from the timeline she's in, would leave her rock solid.

Metal Gear Solid 4 takes this to its logical conclusion with interactive jiggle physics. Shaking the motion-sensitive controller while communicating with a female support character or fighting a female boss outside of her suit causes their breasts to bounce.

Other

Jiggle Physics is Japanese HGame maker Illusion's bread and butter. Depending on which game we're talking about, there may be sliders to adjust the amount of bounciness and the size of the breasts on the female character(s). Depending on the settings you choose, the results can range from subtle and surprisingly realistic to absolutely utterly ridiculous.

The proper scientific term for this technology, and the name it goes by in professional 3D graphics software suites such as Maya, Softimage, Blender, etc.; is "Soft Body Simulation". And now you know...

This feature was a shock and source of amusement to a lot of people discovering Nekopara on Steam, especially as it was the all ages version. There are indeed sliders.

A Japanese development company is looking to make the technology even better. In this VR simulation, the faster you run, the more the Gainax effect in the sprite you are looking at...

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